1. Field of the Invention
The present invention relates to a differential amplifier and to the application thereof to a band-gap voltage generator.
The band-gap voltage (approximately 1.2 volts) is a temperature-independent voltage that is used as a reference voltage in applications requiring high temperature stability.
2. Discussion of the Related Art
FIG. 1 schematically represents a conventional band-gap voltage generator such as described, for example, in "Analysis and Design of Analog Integrated Circuits" by P. R. Gray and R. G. Meyer; John Wiley & Sons.
An operational amplifier 10 amplifies the voltage difference between a node A and a node B and applies the amplified voltage at a node C. Node A is connected to a low voltage Vss successively through a resistor R1 and a diode-connected PNP bipolar transistor Q1, and is connected to node C through a resistor R2. Node B is connected to node C through a resistor R3, and to the low voltage Vss through a diode-connected PNP transistor Q2. The emitter surface of transistor Q1 is larger than the emitter surface of transistor Q2.
The operational amplifier 10, supplied between a high voltage Vdd and the low voltage Vss, acts on node C to keep the voltages of nodes A and B equal. Thus, the voltage across resistor R1 is equal to the difference between the base-emitter voltages of transistors Q1 and Q2. This difference is proportional to the absolute temperature and to the natural logarithm of the ratio of the emitter surface of transistor Q1 to the emitter surface of transistor Q2. This voltage difference, which has a positive temperature coefficient, induces a current that flows across resistor R2 (between nodes C and A), and generates a voltage whose value depends on the ratio R2/R1. Feedback from the operational amplifier 10 forces the voltage between node A and the low voltage Vss to equal the base-emitter voltage of transistor Q2, which has a negative temperature coefficient. By suitably choosing ratio R2/R1, the temperature coefficient of the resulting voltage between node C and Vss is cancelled. In this case, the band-gap voltage is obtained between node C and Vss. The value of resistor R3 is selected to equal the value of resistor R2.
In bipolar technology, there are very simple solutions for implementing such generators, more particularly, for implementing the operational amplifier 10.
In CMOS technology, an operational amplifier satisfying the function of the operational amplifier 10 of FIG. 1 includes a predetermined number of branches, that is, current paths between the high voltage Vdd and the low voltage Vss, which must be biased by a current generator. The simplest solution for biasing the branches consists of providing a biasing generator 12 that feeds a multi-output current mirror, each output serving to bias a branch.
A circuit such as the one of FIG. 1, more particularly the biasing generator 72, has a stable zero-current operating point. Thus, if no current starts flowing through the circuit upon powering-on, amplifier 10, and therefore the voltage generator, fails to start operating. To avoid this drawback, a starting circuit 14 is provided to detect zero current in one branch and to inject a current into the circuit to make the circuit switch to a non-zero current operating point.
A drawback of the CMOS circuits such as the one of FIG. 1 is that a large number of branches are necessary in conventional implementations of operational amplifiers 10. The need for a large number of branches involves both a complex circuit and a particularly high power consumption because, generally, the current in the different branches is of the same order of magnitude.